The invention relates to a seat belt retractor.
U.S. Pat. No. 6,290,159 teaches a seat belt retractor wherein the load limiting effect of a load limiter in the form of a hydraulic pump is adjusted as a function of a measuring signal, which is formed by sampling the rotary motion of the belt reel after blocking of the blockable part of the belt reel. A controllable valve with adjustable flow cross-section may be provided to this end.
A force limiter for a vehicle seat belt is known from DE 22 38 169 which takes the form of a shock absorber with movable shock absorber pistons.
A load limiter is likewise known from U.S. Pat. No. 3,901,345 for a vehicle safety means, in which the damping behavior of a hydraulic damper in the form of a piston/cylinder arrangement is adjusted.
A load limiter in the form of a damping means and constituting a component of a motor vehicle safety system is known from DE 199 63 580 C2, the damping medium of which is a rheological fluid of adjustable viscosity. The level of viscosity of the fluid and thus the damping property of the load limiter may be adjusted by an electrical or magnetic field.
In the present invention, the flow cross-section of the at least one flow channel, through which the load limiting medium is transported by the displacement means, has a constant area, the dimensions of which are selected as a function of the kinematic viscosity of the load limiting medium within certain limits, as will be explained in detail below.
To move the load limiting medium, which comprises an incompressible, free-flowing medium, for example oil, in particular silicone oil, a displacement means, for example a piston, may be provided which is driven by the belt reel via a rack and pinion gear. Moreover, a displacement means may be used that takes the form of a piston movable along the belt reel axis and relative to which the belt reel is rotatable. The displacement means may be effected by the belt reel via a thread or via cutting engagement. Due to the constant flow cross-section of the flow channel, a profile over time of the belt force may in particular be achieved which resembles that generated by a load limiter in the form of a torque rod.